If you have some variable in JS, you might end up needing to convert it to another data type. For example, to change from a string to an integer value. Here is how to do it.

This guide presents some regular ways to convert types in JS. There are quite a few alternative ways (such as `"" + 1` to convert numbers to string).

Converting to a string

Convert From number to string in JS

If you wrap a number in String( some_int_or_float ), it will return a string object.

const numeric_val = 1234;
console.log(numeric_val);
console.log(typeof numeric_val);
const string_from_numeric = String( numeric_val) ;
console.log(string_from_numeric);
console.log(typeof string_from_numeric);

1234

number

1234

string

Alternatively you can just do .toString() on a number variable and it will return a string.

let num = 123;
let num_as_string = num.toString();

Convert from a boolean to a string in JS

To convert from a boolean to a string, it follows a similar pattern as from a number to a string. Just wrap it in String ( true ) and it will return a 4 char string 'true' (or 'false' ).

let your_bool = false;
console.log(false);
console.log(typeof false);
let string_from_your_bool = String( your_bool) ;
console.log(string_from_your_bool);
console.log(typeof string_from_your_bool);

false

boolean

false

string

You can also use the .toString() method:

let your_bool = false;
your_bool_as_string_2 = your_bool.toString();
console.log("Type: " + (typeof your_bool_as_string_2) + " and its value: " + your_bool_as_string_2);

Type: string and its value: false

How to convert from an array to a string in JS

To convert from an array to a string in Javascript, you just wrap an array in String( ['a','b','c'] ) and it will return a comma separated list of the items.

let array = [100,200,333,444];
let array_as_string = String(array);
console.log(array_as_string);
// logs the string "100,200,333,444"

Convert from a date to a string in JS

You can wrap a Data object in String ( new Date ) to get it as a string.

let some_date = new Date;
console.log(some_date);
console.log(typeof some_date);
let string_from_some_date = String( some_date ) ;
console.log(string_from_some_date);
console.log(typeof string_from_some_date);

Wed Jan 12 2016 01:50:10 GMT+0200 (Central European Summer Time)

object

Wed Jan 12 2016 01:50:10 GMT+0200 (Central European Summer Time)

string

How to convert to numbers

How to convert from a string to a number:

To convert from a string (which contains a number) you can do it like this:

let a_number_as_a_string = '1234'; // this is a number
let converted_to_a_number = Number( a_number_as_a_string ); // now it is a number!
// alternative ways to do this:
// as an integer
let int_value = parseInt(a_number_as_a_string);
// as a float:
let float_value = parseFloat(a_number_as_a_string);
let float_value_2 = parseFloat("3.14");

However if you tried to convert a string such as 'asdfg' it would return NaN (not a number) as obviously there is no way to convert 'asdfg' to a number.

Convert a boolean to a number in JS

This will convert bools to numbers. A true value gets converted to 1, and false to 0. Null (which isn't a boolean) also would convert to 0.

let a_boolean = true;
let converted_to_a_number = Number( a_boolean ); // now it is a number! it's value is 1

Automatic type coercion

JS can do some conversion without you having to explicitly tell it to.

For example, in the following code because it starts with a string (a_string), when it gets to the + a_number it will convert a_number to its string value.

let a_string = "Hello, world ";
let a_number = 1234;
console.log(a_string + a_number);

Hello, world 1234